In recent years, the necessity of energy saving has been enhancing the widespread use of electric power converters such as inverters and servo-amplifiers carrying out variable speed driving of motors. Since abnormal operation of electrical devices sometimes results in serious accidents, device makers are required to make the devices provide full safety functions. Moreover, in each of the steps of development, design, production, maintenance and disposal of a product, strict standards are established for restricting dangers to the human body within allowable limits.
In particular, with respect to electrical devices, there are established the functional safety standard of IEC61508 and the standard of IEC61800-5-2 in which safety of motor driven systems such as inverters and servo amplifiers are taken into consideration.
For making the electrical devices meet the safety standards, it becomes necessary for manufacturers of the electrical devices not only to prepare documents with respect to the system and the concept of the development of the devices for receiving certifications as to whether the electrical devices are in conformity with the standards, but also to have the components that are used to satisfy the safety levels of the failure rates. The manufacturers also need to provide a monitoring function that monitors abnormalities of devices relating to safety functions together with a function that safely stops the devices according to specifications.
For stopping a motor, there is a safety torque-off (STO) function that cuts off the supply of energy to the motor. For a related technology with respect to the safety torque-off function, a safety device is described in U.S. Pat. No. 7,253,577, for example. The safety device is provided with a gate circuit (cut off device) between a main circuit that supplies electric power for driving a motor and a control device that produces a control signal for controlling semiconductor switching devices in the main circuit. By closing the gate circuit to cut off the control signal by a cut off instruction from an independently provided safety functional circuit, the supply of the electric power from the main circuit is stopped. The safety functional circuit, as one of the cut off conditions of the gate circuit, outputs the cut off instruction when a safety torque-off instruction (cut off signal) is externally inputted.
The cut off signal, however, being a signal that is not always inputted, makes it not sure in the related cut off device whether the gate circuit operates normally when the cut off signal is actually inputted. An attempt to input a simulated cut off signal for confirming whether the safety torque-off function is normally working or not requires the operation of the electric power converter to be stopped and causes the problem of reducing operating efficiency.